The reason it doesn't stop is this part:

  ...
  do while found()
    replace d1 with dd
  enddo

There is nothing to change the condition, found(), which only tells you whether the last seek() or locate was successful. Therefore, the loop will run without end.

You can check this in the debugger.

>Dear Experts
>
>The following command do nothing and curosr seems to be working for a long time.
>Please make some changes to work
>

>SELECT vou_sum
>GO top
>DO while !eof()
>	AA=date 		&& date type
>	bb=VOU_NO 	&& numueric type
>	cc=ALLTRIM(VOU_TYPE)&& character type
>	dd=d1
>	
>	SELECT VOUCHERS
>	LOCATE for date=AA and VOU_NO=bb and VOU_TYPE=cc
>	DO while FOUND()
>		REPLACE d1 WITH dd
>	ENDDO
>
>	SELECT vou_sum
>	Skip
>ENDDO
>